DOCUMENT RESUME

ED 345 949 SE 053 003

TITLE Our Solar System at a Glance. InformationSummaries.

INSTITUTION National Aeronautics and Space Administration,Washington, D.C.

REPORT NO NASA-PMS-010-A-(JPL)PUB DATE Jun 91NOTE :flp.; Photographs may not reproduce clearly.PUB TYPE Guides - Classroom Use - Instructional Miterials (For

Learner) (051) -- Guides - Classroom Use - TeachingGuides (For Teacher) (052)

EDRS PRICE MF01/PCO2 Plus Postage.DESCRIPTORS Instructional Materials; *Satellites (Aerospace);

*Scientific and Technical Information; *SpaceExploration; *Space Sciences

IDENTIFIERS Comets; Moon; National Aeronautics amd SpaceAdministration; *Planets; Solar System; *Spacecraft;Sun

ABSTRACTThe United States has explored the solar system aith

automated spacecraft and human-crewed expeditions that have produceda quantum leap in our knowlwdge and understanding of the solarsystem. Through the electronic sight and other "senses" of ourautomated spacecraft, color and complexion have been given to worldsthat for centuries appeared to eyes on Earth as fuzzy disks orindistinct points of light end dozens of previously unknown objectshave been discovered. Following a categorical listing of 41 NASAunmanned spacecraft launched between 1959 to 1990, this NASApublication contains numerous photographs and presents factualsummaries of the following: (1) the evolution of automatedspacecraft; (2) current knowledge about the Sun including informationon spacecraft that visited the Sun and future solar missions; (3)conditions on Mercury, especially information that was learned fromthe Mariner 10 spacecraft; (4) information on Venus as gleaned fromMariner 2, Mariner 5, the Pioneer Venus Orbiter, the Pioneer VenusMultiprobe, and Magellan spacecraft; (5) information about the Earthas learned from trips into space; (6) information about the moon, theApollo program, and theories on the origin of th7 moon; (7) detailedinformation on Mars including information from spacecraft on itstopography, prospects for harboring extraterrestrial life, and itstwo moons Phobos and Deimos; (8) research on asteroids and theresults of meteoroid collisions with the earth; (9) data gathered onJupiter and its moons Io, Europa, Ganymede and Callisto; (10) data onSaturn and its rings; (11) information on Uranus, its numerous icymoons including Miranda, Ariel, Umbriel, and Titania; (12)information on Neptune and its moons including Proteus and Nereid;(13) data concerning Pluto our most distant planet and its one moonCharon; and (14) information on the composition and orbits of cometsincluding Halley's comet. A pictorial summary chart gives furtherinformation on the planets including mean distance from tne sun,relative size, periods of revolution and rotation, inclination,eccentricity, and composition of their atmospheres. (PR)

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National Aeronautics andSpace Adrnsnistration

I nformation summaries:

PMS 010-A (JPL)Will June 1991

cl4 Our lar at a Glan

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Points or view or cif:smarts stated al documem do not necessenty represent otticteIOERI positron or poticy

rorn our 3mall world we have gazed upon the

cosmic ocean for untold thousands of years.

Ancient astronomers observed points of light

that appeared to move among the stars. They

called Mese objects planets. meaning wanderers, and

named them after Roman deities .:upiter, king of the

gods; Mars, the god of war; Mercury, messenger of the

gods; Venus, the god of love and beauty, and Saturn.

father of Jupiter and god of agriculture. The stargazers

also observed comets with sparkling tails, and meteors

or shooting stars apparently falling from the sky.

Science flourished during the European Renaissance.

Fundamental physical laws governing planetary motion

were discovered, and the orbits of the planets around the

Sun were calculated. In the 17th century, astronomers

pointed a new device called the telescope at The heavens

and made startling discoveries.

But the years since 1959 have amounted to a golden

age of solar system exploration. Advancements in rocketry

Cover: The nine known planets and the Sun are drawn to scalesize in this artist's rendering of our solar system. If the planetswere actually the sizes shown, the average distancebetween theSun end Pluto would be 550 meters (1,800 feet).

after World War ll enabled our machines to break the grip

of Earth's gravity and travel to the Moon and to other

planets.

The United States hes sent automated spacecraft, then

human-crewed expeditions, to explore the Moon Our

automated machines have orbited and landed on Venus

and Mars; explored the Sun's environment; observed

comets, and mace close-range surveys while flying past

Mercury, Jupiter, Saturn, Uranus and Neptune.

These travelers brought a quantum leap in our knowl-

edge and understanding of the solar system. Through the

electronic sight and other "senses" of our automated

spacecraft, color and complexion have been given to

worlds that for centuries appeared to eyes on Earth as

fuzzy disks or indistinct points of light. And dozens of

previously unknown objects have been discovered.

Future historians will likely view these pioneering flights

through the solar system as some of the most remarkable

achievements of the 20th century

Contents

Automated Spacecraft 5

The Sun 7

Mercury 8

Venus 9

Earth 10

The Moon 12

Mars 13

Asteroids 18

Jupiter 19

Galilean Satellites 21

Saturn 23

Uranus 25

Neptune 27

Pluto 29

Comets 30

Summaries

NASA's Automated Explorationof the Solar System: SpacecraftLaunched Between 1959 and 1990 2

The Planets at a Glance 32

4

NASA's Automated Explorationof the Solar System

Spacecraft Launched Between 1959 and 1990

Pioneer Mariner Ranger

Spacecraft Mission Launch Arrival Results

Pioneer 4 Lunar flyby 3/3/59 3/4159 Collected radiation data near Moon.

Mariner 2 Venus flyby 8/27152 12/14182 First craft to visit another planet.

Mariner 4 Mars flyby 11/28/54 7/14/65 Data showed inhospitable world.

Ranger 9 Lunar impact 3/21/65 3/24/65 Photo resolution as fine as 0.25 meter(10 Inches).

Surveyor 1 Lunar landing 5/30/55 6/2166 Simulated Apollo flight & landing.

Pioneer 7 Solar orbit 8/17/66 N/A Same as Pioneer 6.

Lunar Orbiter 3 f.unar orbit 214167 2/8/67 Mapped surface; examined gravitational field.

Lunar Orbiter 4 Lunar polar orbit 5/487 543/67 Orbit allowed photos of polar regions.

"Spacecraft entered solar orbit immediately after leaving Earth,

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Surveyor

mgct .wLunar Orbiter Viking

Spacecraft Mission Launch Arrival Results

Mariner 5 Venus flyby 6114/67 10/19/67 Confirmed harsh surface conditions

Surveyor 5 Lunar landing 9/8167 9111/67 First chemical analysis of lunar soil.

Pioneer 8 Solar orbit

Pioneer 9 Solar orbit

Mariner 7 Mars flyby

12113167 NIA*

11/8/68 N/A*

3127169 8/5/69

Same as Pioneer 6.

Same as Pioneer 6.

Concentrated on southern polar region.

Pioneer 10 Jupiter flyby 3/2/72 12/3173 First examination of an outer planet.

Mariner 10 Venus flybyMercury flybys

11/3173 2/5/74 First photos of Venus; used Venus' gravity to3/29/74, 9/21174 bend path to Mercury. Three successful flybys& 3/16/75 imaged half of Mercury.

'Spacecraft entered solar orbit immediately atter leaving Earth

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Voyager

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Magellan

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Mars orbit & landing 8/20175 6/19/76 & 7120f76 High-resolution photos; soil sampling.

Helios 2 Solar orbit 1/15176 WA' Continued solar studies of Hellos 1.

Voyager 2 Jupiter flybySaturn flybyUranus flybyNeptune flyby

8/20/77 7/9/798/25/811/241868/25189

Additional observations. Voyager Vssuccess allowed retargeting of Voyager 2at Saturn to point spacecraft to Uranus &Neptune. Flybys extremely successful.

Pioneer VenusMultiprobe

Venus atmospheric 818/78 1219/78probes

Main body & tour probes profiledatmosphere.

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10118/89 12/7/95 Will make long-term observations ofJovian system.Galileo Jupiter orbit &

atmospheric probe

'Spacecraft entered solar orbit immediately after leaving Earth.

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Automated Spacecraft

The National Aeronautics and Space Administration's(NASA's) automated spacecraft for solar system explora-tion come in many shapes and sizes. While they aredesigned to fulfill separate and specific mission objectives,the craft share much in common.

Each spacecraft consists of various scientific instru-ments selected for a particular mission, supported bybasic subsystems for electrical power, trajectory andccientation control, as well as for processing data andcommunicating with Earth.

Electrical power is required to operate the spacecraftinstruments and systems. NASA uses both solar energyfrom arrays of photovoltaic cells and small nuclear genera-tors to power its solar system missions. Rechargeablebatteries are employed for backup and supplementalpower.

Imagine that a spacecraft has successfully journeyedmillions of miles through space to fly but one time near aplanet, only to have its cameras and other sensing instru-ments pointed the wrong way as it speeds past the target!To help prevent such a mishap, a subsystem of smallthrusters is used to control spacecraft.

The thrusters are linked with devices that maintain aconstant gaze at selected stars. Just as Earth's earlyseafarers used the stars to navigate the oceans, space-craft use stars to maintain their bearings in space. With thesubsystem locked onto fixed points of reference, flight

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controllers can keep a spacecraft's scientific instrumentspointed at the target body and the craft's communicationsantennas pointed toward Earth. The thrusters can also beused to fine-tune the flight path and speed of the space-craft to ensure that a target body is encountered at theplanned distance and on the proper trajectory.

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Between 1959 and 1971. NASA spacecraft weredispatched to study the Moon and the solar environment, Aboard FM Atias Centaur rocket, the Mariner 6 spacecraft liftsthey also scanned the inner planets other than Earth away from a launch pad at Cape Canaveral inFlorida.

Mercury, Venus and Mars These three worlds, and ourown, are known as the terrestrial planets because theyshare a sold-rock composition.

For the early planetary reconnaissance missions, NASAemployed a highly successful series of spacecraft calledthe Mariners. Their flights helped shape the planning of

later missions. Between 1962 and 1975. seven Marinermissions conducted the first surveys of our planetaryneighbors in space.

All of the Mariners used solar panels as their primarypower source. The first and the fine; versions of the space-craft had two wings covered with photovoltaic cells. OttlerMariners were equipped with four solar panels extencingfrom their octagonal bodies.

Although the Mariners ranged from the Mariner 2 Venusspacecraft, weighing in at 203 kilograms (447 pounds), tothe Mariner 9 Mars Orbiter, weighing in at 974 kilograms

(2,147 pounds), their basic design remained quite similarthroughout the program. The Mariner 5 Venus spacecraft,for example, had originally been a backup for the Mariner 4Mars flyby. The Mariner 10 spacecraft sent to Venus andMercury used components left over from the Mariner 9Mars Orbiter program.

In 1972, NASA launched Pioneer 10, a Jupiter space-craft. Interest was shifting to fow of the outer planetsJupiter, Saturn, Uranus and Neptune giant balls ofdense gas quite different from the terrestrial worlds we hadalready surveyed.

Four NASA spacecraft in all two Pioneers and twoVoyagers were sent in the 1970s to tour the outerregions of our solar system. Because of the distancesinvolved, these travelers took anywhere from 20 months to12 years to reach their destinations. Barring faster space-craft, they will eventually become the first human artifactsto journey to distant stars. Because the Sun's light be-comes so faint in the outer solar system, these travelers donot use solar power but instead operate on electricitygenerated by heat from the decay of radioisotopes.

NASA also developed highly specialized spacecraft torevisit our neighbors Mars and Venus in the middle andlate 1970s. Twin Viking Landers were equipped to serve asseismic and weather stations and as biology laboratories.Two advanced orbiters descendants of the Mariner craft

carried the Viking Landers from Earth and then studiedmartian features from above

Two drum-shaped Pioneer spacecraft visited Venus in1978. The Pioneer Venus Orbiter was equipped with aradar instrument that allowed it to "'see" through theplanet's dense cloud cover to study surface features. ThePioneer Venus Multiprobe carried four probes that weredropped through the clouds. The probes and the mainbody all of which contained scientific instrumentsradioed information about the planet's atmosphere duringtheir descent toward the surface.

A new generation of automated spacecraft includngMagellan, Galileo, Ulysses, Mars Observer, the CometRendezvous/Asteroid Flyby (CRAF) and Cassini isbeing developed and sent out into the solar system tomake detailed examinations that will increase our under-standing of our neighborhood and our own planet.

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Tests in a space simulator prepared the Galileo Orbiter for itssix-year Journey to Jupiter.

The Sun

A discussion of the objects in the solar system must startwith the Sun. The Sun dwarfs the other bodies, represent-ing approximately 99.86 percent of all the mass in the solarsystem; all of the planets, moons, asteroids, comets,dust and gas add up to only about 0.14 percent. This0.14 percent represents the material left over from theSun's formation. One hundred and nine Earths would berequired to fit across the Sun's disk, anql its interior couldhold over 1.3 million Earths.

As a star, the Sun generates energy by the processof fusion. The temperature at the Sun's core is 15 milliondegrees Celsius (27 million degrees Fahrenheit), and thepressure there is 340 billion times Earth's air pressure atsea level. The Sun's surface temperature of 5,500 degreesCelsius (10,000 degrees Fahrenheit) seems almost chillycompared to its core temperature! At the solar core,hydrogen can fuse into helium, producing energy. The Sunalso produces a strong magnetic field and streams ofcharged particles, the field and streams extending farbeyond the planets.

The Sun appears to have been active for 4.6 billionyears and has enough fuel for another five billion years

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Turbulent helium gas on end near the Sun is visible In thisultraviolet-light photograph taken by astrensuis on Skykrb,an Earth-orbiting space station, in June 1973.

or so. At the end of its life, the Sun will start to fusehelium into heavier elements and begin to swell up,ultimately growing so large that it will swallow Earth_After a billion years as a "red giant," it will suddenlycollapse into a "white dwarf" the final end productof a star like ours. It may take a trillion years to cool offcompletely,

Many spacecraft have explored the Sun's environ-ment, but none have gotten any closer to its surfacethan approximately two-thirds of the distance from Earthto the Sun. Pioneers 5-11, the Pioneer Venus Orbiter,Voyagers 1 and 2 and other spacecraft have all sam-pled the solar environment. The Ulysses spacecraft,launched on October 6,1990, is a joint solar mission ofNASA and the European Space Agency. After usingJupiter's gravity to change its trajectory, Ulysses will flyover the Sun's polar regions during 1994 and 1995 andwill perform a wide range of studies using nine onboardscientific instruments,

We are fortunate that the Sun is exactly the way it is.If it were different in almost any way. life would almostcertainty never have developed on Erth

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Mercury

Obtaining the first close-up views of Mercury was theprimary objective of the Mariner 10 spacecraft, launchedon November 3, 1973, from Kennedy Space Center inFlorida. After a journey of nearly five months, including aflyby of Venus, the spacecraft passed within 703 kilometers(437 miles) of the solar system's innermost planet onMarch 29, 1974.

Until Mariner 10, little was known about Mercury. Eventhe best telescopic views from Earth showed Mercury asan indistinct object lacking any surface detail. The planetis so close to the Sun that it is usually lost in solar glare.When the planet is visible on Earth's horizon just after sun-set or before dawn, it is obscured by the haze and dustin our atmosphere Only radar telescopes gave any hint ofMercury's surface conditions prior to the voyage of Mari-ner 10.

The photographs Mariner 10 radioed back to Earth re-vealed an ancient, heavily cratered surface, closely resem-bling our own Moon. The pictures also showed huge cliffscrisscrossing the planet. These apparently were createdwhen Mercury's interior cooled and shrank, buckling theplanet's crust. The cliffs are as high as 3 kilometers (2 miles)and as long as 500 kilometers (310 miles).

Instruments on Mariner 10 discovered that Mercury hasa weak magnetic field and a trace of atmosphere atrillionth the density of Earth's atmosphere and composedchiefly of argon, neon and helium. When the planet's orbittakes it closest to the Sun, surface temperatures rangefrom 467 degrees Celsius (872 degrees Fahrenheit) onMercury's sunlit side to -183 degrees Celsius (-298 degreesFahrenheit) on the dark side. This range in surface temper-ature 650 degrees Celsius (1,170 degrees Fahrenheit)

is the largest for a single body in the solar system.Mercury literally bakes and freezes at the same time.

Days and nights are long on Mercury. The combinationof a slow rotation relative to the stars (59 Earth days) and arapid revolution around the Sun (88 Earth days) means thatone Mercury solar day takes 176 Earth days or two Mercu-ry years the time it takes the innermost planet to com-plete two orbits around the Sun!

Mercury appears to have a crust of light silicate rock likethat of Earth. Scientists believe Mercury has a heavy iron-rich core making up slightly less than half of its volume.That would make Mercury's core larger, proportionally,than the Moon's core or those of any of the planets.

After the initial Mercury encounter, Mariner 10 made twoadditional flybys on September 21, 1974, and Match 16,1975 before control gas used to orient the spacecraftwas exhausted and the mission was concluded. Each flybytook place at the same local Mercury time when theidentical half of the planet was i'luminated: as a result, westill have not seen one-half of the planet's surface.

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Venus

Veiled by dense cloud cover, Venus our nearestplanetary neighbor was the first planet to be explored.The Mariner 2 spacecraft, launched on August 27, 1962,

was the first of more than a dozen successful Americanand Soviet missions to study the mysterious planet. Asspacecraft flew by or orbited Venus, plunged into the atrno-sphere of gently landed on Venus' surface, romantic mythsand speculations about our neighbor were laid to rest.

On December 14, 1962, Mariner 2 passed within34,839 kilometers (21,648 miles) of Venus and becamethe first spacecraft to scan another planet: onboardinstruments measured Venus for 42 minutes. Mariner 5,

launched in June 1967, flew much closer to the planet.Passing within 4,094 kilometers (2,544 miles) of Venus onthe second American flyby, Mariner 5's instrumentsmeasured the planet's magnetic field, ionosphere, radia-tion belts and temperatures. On its way to Mercury, Mari-ner 10 flew by Venus and transmitted ultraviolet pictures toEarth showing cloud circulation patterns in the Venusian

atmosphere.ln the spring and summer of 1978, two spacecraft were

launched to further unravel the mysteries of Venus. OnDecember 4 of the same year, one of these travelers thePioneer Venus Orbiter became the first spacecraftplaced in orbit around the planet.

The circulation of Venus' dense ehnosphere Is discernible in thisenhanced photograph from the Pioneer Venus Orbiter.

Five days later, the five separate components makingup the second spacecraft the Pioneer Venus Multiprobe

entered the Venusian atmosphere at different locationsabove the planet. The four small, independent probesandthe main body radioed atmospheric data back to Earthduring their descent toward the surface. Although de-signed to examine the atmosphere, one of the probessurvived its impact with the surface and continued totransmit data for another hour.

Venus resembles Earth in size, physical compositionand density more closely than any other known planet.However, spacecraft have discovered significant differ-ences as well. For example, Venus' rotation (west to east)is retrograde (backward) compared to the east-to-westspin of Earth and most of the other planets.

Approximately 96.5 percent of Venus atmosphere(95 times as dense as Earth's) is carbon dioxide. Theprincipal constituent of Earth's atmosphere is nitrogen.Venus' atmosphere acts like a greenhouse, permitting solarradiation to reach the surface but trapping the heat thatwould ordinahly be radiated back into space As a result,

the planet's average surface temperature is 482 degreesCelsius (900 degrees Fahrenheit), hot enough to melt lead.

A radio altimeter on the Pioneer Venus O-.'ter providedthe first means of seeing through the planet .5.1ense cloudcover and determining surface features oyes climost theentire planet. NASA's Magellan spacecraft, launched onMay 5, 1989, has orbited Venus since August 10, 1990.

The spacecraft uses radar-mapping techniques to provideultrahigh-resolution images of the surface.

Magellan has revealed a landscape dominated byvolcanic features, faults and impact craters. Huge areas of

the surface show evidence of multiple periods of lavaflooding with flows lying on top of previous ones. Anelevated region named Ishtar Terra is a lava-filled basin aslarge as the United States. At one end of this plateau sitsMaxwell Montes, a mountain the size of Mount Everest,Scarring the mountain's flank is a 100-kilometer (62-mile)wide, 2.5-kilometer (1.5-mile) deep impact crater namedCleopatra. (Almost all features on Venus are named for

women; Maxwell Montes, Alpha Regio and Beta fi *gio arethe exceptions.) Craters survive on Venus for perhaps400 million years because there is no water and very little

wind erosion.Extensive fault-hne networks cover the planet, probably

the result of the same crustal flexing that produces platetectonics on Earth. But on Venus the surface temperatureis sufficient to weaken the rock, which cracks just abouteverywhere, preventing the formation of major plates andlargr: earthquake faults like the San Andreas Fault in

California.Venus' predominant weather pattern is a high-altitude.

high-speed circulation of clouds that contain sulfuric acid

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At speeds reaching as high as 360 kilometers (225 miles)per hour, the clouds circle the planet in only four Earthdays. The circulation is in the same direction west toeast as Venus' slow rotation of 243 Earth days, whereasEarth's winds blow in both directions west to east andeast to west in six alternating bands. Venus' atmosphereserves as a simplified laboratory for the study of ourweather.

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Three large impact craters and numerous faults appear I n thisMagellan radar mosaic of Venus surface. The lamest aster hasa diameter of 59 kilometers (31 miles). The entire area show is550 kilometers (342 miles) wide and SOO kilometers (310 miles)long.

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Earth

As viewed from space, our world's distinguishing charac-teristics are its blue waters, brown and green land massesand white clouds We are enveloped by an ocean of airronsisting of 78 percent nitrogen, 21 percent oxygen and1 percent other constituents. The only planet in the solarsystem known to harbor life, Earth orbits the Sun at anaverage distance of 150 million kilometers (93 millionmiles). Earth is the third planet from the Sun and the fifthlargest in the solar system, with a diameter just a fe..vhundred kilometers larger than that of Venus.

Our planet's rapid spin and molten nickel-iron core giverise to an extensive magnetic field, which, along with theatmosphere, shields us from nearly all of the harmfulradiation coming from the Sun and other stars. Earth'satmosphere protects us from meteors as well, most ofwhich burn up before they .'an strike the surface. Activegeological processes hav,.: lett no evidence of the peltingEarth almost certainly received soon after it formedabout 4.6 billion years ago. Along with the other newlyformed planets. it was showered by space debris in theearly days of the solar system.

From our journeys into space, we have learned muchabout our home planet. The first American satelliteExplorer 1 was launched from Cape Canaveral inFlorida on January 31, 1958, and discovered an intenseradiation zone, now called the Van Allen radiation belts,surrounding Earth.

Since then, other research satellites have revealed thatour planet's magnetic field is distorted into a tear-dropshape by the solar wind the stream of charged particlescontinuously ejected from the Sun. We've learned that themagnetic field does not fade off into space but has definiteboundaries. And we now know that our wispy upperatmosphere, once believed calm and uneventful, seetheswith activity swelling by day and contracting by night.Affected by changes in solar activity, the upper atmo-sphere contributes to weather and climate on Earth.

Besides affecting Earth's weather, solar activity givesrise to a dramatic visual phenomenon in our atmosphere.When charged particles from the solar wind becometrapped in Earth's magnetic field, they collide with airmolecules above our planet's magnetic poles. These airmolecules then begin to glow and are known as theauroras or the northern and southern lights.

Satellites about 35,789 kilometers (22,238 miles) out inspace play a major role in daily local weather forecasting.These watchful electronic eyes warn us of dangerousstorms. Continuous global monitoring provides a vastamount of useful data and contributes to a better under-standing of Earth's complex weather systems.

Frorn their unique vantage points, satellites can surveyEarth's oceans, land use and resources, and monitor theplanet's health. These eyes in space have saved countlesslives, provided tremendous conveniences and shown usthat we may be altering our planet in dangerous ways.

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The Moon

The Moon is Earth's single natural satellite. The first humanfootsteps on an afien world were made by Americanastronauts on the dusty surface of our airless, lifelesscompanion. In preparation for the human-crewed Apolloexpeditions, NASA dispatched the automated Ranger,Surveyor and Lunar Orbiter spacecraft to study the Moonbetween 1964 and 1968.

NASA's Apollo program left a large legacy of lunarmaterials and data. Six two-astronaut crews landed on andexplored the lunar surface between 1969 and 1972,carrying back a collection of rocks and soil weighing a totalof 382 kilograms (842 pounds) and consisting of more than2,000 separate samples.

From this material and other studies, scientists haveconstructed a history of the Moon that includes its infancy.Rocks collected from the lunar highlands date to about4.0-4.3 billion years old. The first few million years of theMoon's existence were so violent that few traces of thisperiod remain. As a molten outer layer gradually cooledand solidified into different kinds of rock, the Moon %If,bombarded by huge asteroids and smaller objects. Someof the asteroids were as large as Rhode Island or Dela-ware, and their collisions with the Moon created basinshundreds of kilometers across.

This catastrophic bombardment tapered off approxi-mately four billion years ago, leaving the lunar highlandscovered with huge. overlapping craters and a deep layerof shattered and broken rock. Heat produced by the decayof radioactive elements began to melt the interior of theMoon at depths of about 200 kilometers (125 miles) belowthe surface. Then, for the next 700 million years fr9mabout 3.8 to 3.1 billion years ago lava rose from insidethe Moon. The lava gradually spread out over the surface,flooding the large impact basins to form the dark areas thatGalileo Oa lilei, an astronomer of the Italian Renaissance,called maria, meaning seas.

As far as we can tell, there has been no significantvolcanic activity on the Moon for more than three billionyears. Since then, the lunar :urface has been altered onlyby microrneteorites, by the atomic particles from the Sunand stars, by the rare impacts of large meteorites and byspacecr aft and astronauts. If our astronauts had landed onthe Moon a billion years ago, they would have seen alandscape very similar to the one today. Thousands ofyears from now, the footsteps left by the Apollo crews willremain sharp and clear.

The origin of the Moon is still a mystery. Four theoriesattempt an explanation: the Moon formed near Earth as aseparate body: it was torn from Earth: it formed somewhereelse and was captured by our planet's gravity, or it was the

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result of a collision between Earth and an asteroid aboutthe size of Mars. The last theory has some good supportbut is far from certain.

Apollo 12 astrmnarts &Wed on the lunar surface within walkkgdistance of Surveyor 3 a feat made possible by the photographicskill of Lunar Orbiter 3.

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Mars

Of all the planets, Mars has long been considered thesolar system's prime candidate for harboring extraterres-trial life. Astronomers studying the red planet throughtelescopes saw what appeared to be straight lines criss-crossing its surface. These observations later deter-mined to be optical illusions led to the popular notionthat intelligent beings had constructed a system of irriga-tion canals on the planet. In 1938, when Orson Wellesbroadcast a radio drama based on the science fictionclassic War of the Worlds by H.G. Wells, enough peoplebelieved in the tale of invading martians to cause a nearpanic.

Another reason for scientists to expect life on Mars hadto do with the apparent seasonal color changes on theplanet's surface. This phenomenon led to speculation thatconditions might support a bloom of martian vegetationduring the warmer months and cause plant life to becomedormant during colder periods.

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So far, six American missions to Mars have been carriedout. Four Mariner spacecraft three flying by the planetand one placed into martian orbit surveyed the planetextensively before the Viking Orbiters and Landers arrived.

Mariner 4, launched in late 1964, fiew past Mars onJuly 14, 1965, within 9,846 kilometers (6,118 miles) of thesurface. Transmitting to Earth 22 close-up pictures of theplanet, the spacecraft found many craters and naturallyoccurring channels but no evidence of artificial canals orflowing water. Mariners 6 and 7 followed with their flybysduring the summer of 1969 and returned 201 pictures.Mariners 4, 6 and 7 showed a diversity of surface condi-tions as well as a thin, cold, dry atmosphere of carbondioxide.

On May 30, 1971. the Mariner 9 Orbiter was launchedon a mission to make a year-long study of the martiansurface. The spacecraft arrived five and a half months afterliftoff, only to find Mars in the midst of a planet-wide dust

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storm that made surface photography impossible forseveral weeks. But after the storm cleared, Mariner 9began returning the first of 7,329 pictures; these revealedpreviously unknown martian features, including evidencethat large amounts of water once flowed across thesurface, etching river valleys and flood plains.

In August and September 1975, the Viking 1 and 2spacecraft each consisting of an orbiter and a lenderlifted off from Kennedy Space Center. The mission wasdesigned to answer several questions about the redplanet, including, Is there life there? Nobody expected thespacecraft to spot martian cities, but it was hoped that thebiology experiments on the Viking Landers would at leastfind evidence of primitive life past or present.

Viking Lander 1 became the first spacecraft to success-fully touch down on another planet when it landed onJuly 20, 1976, while the United States was celebrating itsBicentennial. Photographs sent back from Chryse Planitia("Plains of Gold") showed a bleak, rusty-red landscape.Panoramic images returned by Viking Lander 1 revealed arolling plain, littered with rocks and marked by rippled sanddunes. Fine red dust from the martian soil gives the sky asalmon hue. When Viking Lander 2 touched down onUtopia Planitia on September 3, 1976, it viewed a morerolling landscape than the one seen by its predecessorone without visible dunes.

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The results sent back by the laboratory on each VikingLander were inconclusive. Small samples of the redmartian soil were tested in three different experimentsdesigned to detect biological processes. While some of thetest results seemed to indicate biological activity, lateranalysis confirmed that this activity was inorganic in natureand related to the planet's soil chemistry. Is there life onMars? No one knows for sure, but the Viking mission foundno evidence that organic molecules exist there.

The Viking Landers became weather stations, recordingwind velocity and direction as well as atmospheric temper-ature and pressure. Few weather changes were observed.The highest temperature recorded by either spacecraftwas -14 degrees Celsius (7 degrees Fahrenheit) at theViking Lander 1 site in midsummer.

The lowest temperature, -120 degrees Celsius (-184 de-grees Fahrenheit), was recorded at the more northerlyViking Lander 2 site during winter. Near-hurricane windspeeds were measured at the two martian weather stationsduring global dust storms, but because the atmosphere isso thin, wind force is minimal. Viking Lander 2 photo-graphed light patches of frost probably water-iceduring its second winter on the planet.

The martian atmosphere, like that of Venus, is primarilycarbon dioxide. Nitrogen and oxygen are present only insmall percentages. Martian air contains only aboi ',/1.000

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Light patches of water-ice frost on Utopia Manilla, VikingLander 2's landing site, were observed by the spacecraft duringthe martian winter.

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These martian channels (A) wind their way northwest (upper left)for several hundred kilometers. The origin of the channels iscontroversial: Oki fhey form from lava flows or from waterreleased by the melting of ground ire during volcanic eruptions?The martian channels resemble these dry channels (B) in thecoastal deceit of Peru.

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impact craters are formed when a planetary surface is struck by ameteorite. Marcum our Moon and many of the icy, rocky satellitesof the outer solar system are charecterized by heavily miteredsurfaces. Earth's geological processes tend to destiny evidenceof ancient crater imparts, although some mom recent cratersremain discernible. Compare martian craters (A) with those on theMoon (B).

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Volcanoes are vents in a t's or moon's crust penellting theescape of Internal heat. Earth has hundreds of active volcanoes,like those on the island of Hawaii (A). Hawaiian volcanoes are varysimilar in shape to Mars' extbet Olympus Mons (0), which Is threetimes the height of Mount &ergot and has a base that wouldcover the state of Nevada Tim most volcanically active body inthe soar system is Jupiter's moon lo (C). hi this photographicmosaic of the moon, a cloud of , orejected material (top), Isvisible 280 kilometers (175 miles) above the surface, hurled thereby the violent volcano Pete (highlighted).

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Racks litter the rolling martian Amdscape seen by Viking Lander 1.The large boulder (left), named Ng Joe, measutes 1 by 3 meters(3 by 10 feet) and is 8 meters (26 feet) from the spacecraft Thevertical while oNect (near center) is part of the spacecraft'smeteorological boon .

as much water as our air, but even this small amount cancondense out, forming clouds that ride high in the atmo-sphere or swirl around the slopes of towering volcanoes.Local patches of early morning fog can form in valleys.

There is evidence that in the past a denser martianatmosphere may have allowed water to flow on the planet.Physical features closely resembling shorelines, gorges,riverbeds and islands suggest that great rivers oncemarked the planet.

Mars has two moons, Phobos and Deimos. They aresmall and irregularly shaped and possess ancient,cratered surfaces. It is possible the moons were originallyasteroids that ventured too close to Mars and werecaptured by its gravity.

The Viking Orbiters and Landers exceeded by largemargins their design lifetimes of 120 and 90 dal's, respec-tively. The first to fail was Viking Orbiter 2, whi ;h stoppedoperating on July 24, 1978, when a leak depleted itsattitude-control gas. Viking Lander 2 operated untilApril 12, 1980, when it was shut down due to batterydegeneration. Viking Orbiter 1 quit on August 7, 1980.when the last of its attitude-control gas was used up.Viking Lander 1 ceased functioning on November 13,1983.

Despite the inconclusive results of the Viking biologyexperiments, we know more about Mars than any otherplanet except Earth NASA's Mars Observer spacecraft, tobe launched in September 1992, will expand our knowl-edge of the martian environment and lead to humanexploration of the red planet.

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Mars' Valles Aferinerts (Mariner Valley) woad stretch from thePacific to the Atiantk Oceans, swellowAig up the Grand Canyon.Appearing Me a long scar across the face of Afars, the valley nosformed when lava from volcanoes drained out of a reservok belowthe surfece and the crust collapsed.

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Ph the larger of Afars' two tiny moons, is the first asteroid-like object seen up close. it might be a typical asteroid that wastrapped by Mars' gravity.

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Asteroids

The solar system has a large number of rocky and metallicobjects that are in orbit around the Sun but are too small tobe considered fu.l-fledged planets. These objects areknown as asteroids or minor planets. Most, hut not all, arefound in a band or belt between the crbits of Mars andJupiter. Some have orbits that cross Earth's path, and thereis evidence that Earth has been hit by asteroids ;n the past.One of the least eroded. best preserved examples is theBarringer Meteor Crater near Winslow, Arizona.

Asteroids are material left over from the formation of thesolar system. One theory suggests that they are theremains of a planet that was destroyed in a massivecollision long ago. More likely, asteroids are material thatnever coalesced into a planet. In fact, if the estimated totalmass of all asteroids was gathered into a single object, theobject would be only about 1.500 kilometers (932 miles)across less than half the diameter of our Moon.

Thousands of asteroids have been identified from Earth.it is estimated that 100,000 are bright enough to eventuallybe photographed through Earth-based telescopes.

Much of our understanding about asteroids comes fromexamining pieces of space debris that fall to the surface ofEarth. Asteroids that are on a collision course with Earthare called meteoroids. When a meteoroid strikes ouratmosphere at high velocity, friction causes this chunk ofspace matter to incinerate in a streak of light known as ameteor. If the meteoroid does not burn up completely,what's left strikes Earth's surface and is called a meteorite.One of the best places to look for meteorites is the ice capof Antarctica.

Of all the meteorites examined, 92.8 percent are com-posed of silicate (stone). and 5.7 percent are composed ofiron and nickel; the rest are a mixture of the three materials.Stony meteorites are the hardest to identify since they lookvery mue' . like terrestrial rocks.

Since asteroids are material from the very early solarsystem, scientists are interested in their composition.Spacecraft that have flown through the asteroid belt havefound that the belt is really quite empty and that asteroidsare separated by very large distances.

Current and future missions will fly by selected asteroidsfor closer examination. The Galileo Orbiter, launched byNASA in October 1989, will investigate main-belt asteroidson its way to Jupiter. The Comet Rendezvous/AsteroidFlyby (CRAF) and Cassini missions will also study thesefar-flung, objects Scheduled for launch in the latter part ofthe 1990s, the CRAF and Cassini missions are a collabora-tive project of NASA, the European Space Agency and thefederal space agencies of Germany and Italy, as well asthe United States Air Force and the Department of EnergyOne day, space factories will mine the asteroids for rawmaterials.

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A nendezvous is shown in this artist'sconception of a possible future mission.

Jupiter

Beyond Mars and the asteroid belt, in the outer regions ofour solar system, lie the giant planets of Jupiter, Saturn,Uranus and Neptune. In 1972, NASA dispatched the firstof four spacecraft slated to conduct the initial surveys ofthese colossal worlds of gas and their moons of ice androck. Jupiter was the first port of call.

Pioneer 10, which lifted off from Kennedy Spacc Centerin March 1972, was the first spacecraft to penetrate theasteroid belt and travel to the outer regions of the solarsystem. In December 1973, it returned the first close-upimages of Jupiter, flying within 132,252 kilometers(82,178 miles) of the planet's banded cloud tops. Pio-neer 11 followed a year later. Voyagers 1 and 2 werelaunched in the summer of 1977 and returned spectacularphotographs of Jupiter and its family of satellites duringflybys in 1979.

These travelers found Jupiter to be a whirling ball ofliquid hydrogen and helium, topped with a colorful atmo-sphere composed mostly of gaseous hydrogen andhelium. Ammonia ice crystals form white Jovian clouds.Sulfur compounds (and perhaps phosphorus) mayproduce the brown and orange hues that characterizeJupiter's atmosphere.

It is likely that methane, ammonia, water and othergases react to form organic molecules in the regionsbetween the planet's frigid cloud tops and the warmerhydrogen ocean lying below. Because of Jupiter's atmo-spheric dynamics, however, these organic compoundsif they exist are probably short-lived.

The Great Red Spot has been observed for centuriesthrough telescopes on Earth. This hurricane-like storm inJupiter's atmosphere is more than twice the size of ourplanet As a high-pressure region, the Great Red Spotspins in a direction opposite to that of low-pressure stormson Jupiter; it is surrounded by swirling currents that rotatearound the spot and are sometimes consumed by it. TheGreat Red Spot might be a million years old.

Our spacecraft detected lightning in Jupiter's upperatmosphere and observed auroral emissions similar toEarth's northern lights at the Jovian polar regions. Voy-ager 1 returned the first images of a faint, narrow ringencircling Jupiter.

Largest of the solar system's planets, Jupiter rotates at adizzying pace once every 9 hours 55 minutes 30 sec-onds. The massive planet takes almost 12 Earth years tocomplete a journey around the Sun. With 16 known moons,Jupiter is something of a miniature solar system.

A new mission to Jupiter the Galileo Project isunder way. After a six-year cruise that takes the GalileoOrbiter once past Venus, twice past Earth and the Moonand once past two asteroids, the spacecraft will drop anatmospheric probe into Jupiter's cloud layers and relaydata back to Earth. The Galileo Orbiter will spend two yearscircling the planet and flying close to Jupiter's largemoons, exploring in detail what the two Pioneers and twoVoyagers revealed.

Jupiter Nis Ore sky for toe Voyager I spiceeraft and the moons to(left) end Eur 7pa. to is passing over the Great Red Spot.

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Jupiters very thin ring is seen by Voyager 2, This mosaic ofImages was taken while the spacecraft was In the planers shadow.

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Galilean Satellites

In 1610, Galileo Galilei aimed his telescope at Jupiter andspotted four points of light orbiting the planet. For the firsttime, humans had seen the moons of another world. Inhonor of their discoverer, these four bodies would becomeknown as the Galilean satellites or moons. But Galileomight have happily traded this honor for one look at thedazzling photographs returned by the Voyager spacecraftas they flew past these planet-sized satellites.

One of the most remarkable findings of the Voyagermission was the presence of active volcanoes on theGalilean moon to. Volcanic eruptions had never beforebeen observed on a world other than Earth. The Voyagercameras identified at least nine active volcanoes on lo, withplumes of ejected material extending as far as 280 kilome-ters (175 miles) above the moon's surface.

lo's pizza-colored terrain, marked by orange and yellowhues, is probably the result of sulfur-rich materials broughtto the surface by volcanic activity. Volcanic activity on thissatellite is the result of tidal flexing caused by the gravita-tional tug-of-war between lo, Jupiter and the other threeGalilean moons.

Europa, approximately the same size as our Moon, isthe brightest Galilean satellite. The moon's surface displaysa complex array of streaks, indicating the crust has been

Active end dormant volcanoes dot to's landscape. The light ring(center) is formed by material erupting from the vokanoPrometheus; the ejecta cloud (left) is from Pole.

fractured. Caught in a gravitational tug-of-war like lo,Europa has been heated enough to cause its interior ice tomelt apparently producing a liquid-water ocean. Thisocean is covered by an ice crust that has formed wherewater is exposed to the cold of space. Europa's core ismade of rock that sank to its center.

Like Europa, the other two Galilean moons Gany-mede and Callisto are worlds of ice and rock. Gany-mede is the largest satellite in the solar system largerthan the planets Mercury and Pluto. The satellite is com-posed of about 50 percent water or ice and the rest rock.Ganyrnede's surface has areas of different brightness,indicating that, in the past, material oozed out of themoon's interior and was deposited at various locations onthe surface.

Callisto, only slightly smaller than Ganyrnede, has thelowest density of any Galilean satellite, suggesting thatlarge amounts of water are part of its composition. Callistois the most heavily cratered object in the solar system; noactivity during its history has erased old craters exceptmore impacts.

Detailed studies of all the Galilean satellites will beperformed by the Galileo Orbiter.

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Ganymede's lighter areas have been active more receotty thanthe darker regions. The small, bright spots are the result otimpacts exposing fresh, clean Ice.

No spot on Callisto's surface has escaped bombardment, asshown in this enhanced view (left).

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Saturn

No planet in the solar system is adorned like Saturn. Itsexquisite ring system is unrivaled. Like Jupiter, Saturn iscomposed mostly of hydrogen. But in contrast to the vividcolors and wild turbulence found in Jovian clouds, Saturn'satmosphere has a more subtle, butterscotch hue, and itsmarkings are muted by high-altitude haze. Given Saturn'ssomewhat placid-lcoking appearance, scientists weresurprised at the high-velocity equatorial jet stream thatblows some 1,770 kilometers (1,100 miles) per hour.

Three American spacecraft have visited Saturn. Pio-neer 11 sped by the planet and its moon Titan in Septem-ber 1979, returning the first close-up images. Voyager 1followed in November 1980, sending back breathtakingphotographs that revealed for the first time the complexitiesof Saturn's ring system and moons. Voyager 2 flew by theplanet and its moons in August 1981.

The rings are composed of countless low-densityparticles orbiting individually around Saturn's equator atprogressive distances from the cloud tops. Analysis ofspacecraft radio waves passing through the rings showedthat the particles vary widely in size, ranging from dust tohouse-sized boulders. The rings are bright because theyare mostly ice and frosted rock.

The rings might have resulted when a moon or apassing body ventured too close to Saturn. The unluckyobject would have been torn apart by great tidal forces onits surface and in its interior. Or the object may not havebeen fully formed to begin with and disintegrated under theinfluence of Saturn's gravity. A third possibility is that theobject was shattered by collisions with larger objectsorbiting the planet.

Unable either to form into a moon or to drift away fromeach other, individual ring particles appear to be held inplace by the gravitational pull of Saturn and its satellites.These complex gravitational interactions form the thou-sands of ringlets that make up the major rings.

Radio emissions quite similar to the static heard on anAM car radio during an electrical storm were detected bythe Voyager spacecraft. These emissions are typical oflightning but are believed to be coming from Saturn's ringsystem rather than its atmosphere, where no lightning wasobserved. As they had at Jupiter, the Voyagers saw aversion of Earth's auroras near Saturn's poles.

The Voyagers discovered new moons and found severalsatellites that share the same orbit. We learned that somemoons shepherd ring particles, maintaining Saturn's ringsand the gaps in the rings. Saturn's 18th moon was discov-

_ ered in 1990 from images taken by Voyager 2 in 1981.Voyager 1 determined that Titan has a nitrogen-based

atmosphere with methane and argon one more likeEarth's in composition than the carbon dioxide atmo-spheres of Mars and Venus. Titan's surface temperature of-179 degrees Celsius (-290 degrees Fahrenheit) impiies

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Shadowy "spokes" appear to be very line particles Med slightlyout of the of the rings by Saturn's mimetic field

High, thin of hare rise above Titan's main cloud deck. Thehighest layer is about 700 kilometers (435 miles) above themoon's surface.

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Saturn's cloud bands, rings and some of its 18 moons (above) arevisible to the appmaching Voyager 2 Gaps In the rings can beseen.

Saturn's rings vary in brightness; these variations correspond tothe size and thickness of the ring particles. The dark grey region(right) and the thinner grey band running from the upper left to thelower right (known as the Cassini Division) have only a thin Layerof dust-size particles.

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An ice bail about 500 kilomeferw (310 miles) in diameter, Saturn'smoon Enceladus has had Aolsodes in which sections of II havebeen resudaced.

that there might be water-ice islands rising above oceansof ethane-methane liquid or sludge. Unfortunately, Voy-ager 1's cameras could not penetrate the moon's denseclouds.

Continuing photochemistry from solar radiation may beconverting Titan's methane to ethane, acetylene and in

combination with nitrogen hydrogen c janide. The lattercompound is a building block of amino acids. Theseconditions may be similar to the atmospheric conditions ofprimeval Earth between three and four billion years ago.However, Titan's atmospheric temperature is believed tobe too low to permit progress beyond this stage of organicchemistry.

The exploration of Saturn will continue with the Cassinimission. The Cassini spacecraft will orbit the planet andwill also deploy a probe called Huygens, which will bedropped into Titan's atmosphere and fall to the surface.Cassini will use the probe as well as radar to peer throughTitan's clouds and will spend years examining the Satur-nian system.

Uranus

In January 1986, four and a half years after visiting Saturn,Voyager 2 completed the first close-up survey of theUranian system. The brief flyby revealed more informationabout Uranus and its retinue of icy moons than had beengleaned from ground observations since the planet'sdiscovery over two centuries ago by the English astronomerWilliam Herschel.

Uranus, third largest of the planets, is an oddball of thesolar system. Unlike the other planets (with the exception ofPluto), this giant lies tipped on its side with its north andsouth poles alternately facing the Sun during an 84-yearswing around the solar system. During Voyager 2's flyby,the south pole faced the Sun. Uranus might have beenknocked over when an Earth-sized obiect collided with itearly in the life of the solar system.

Voyager 2 discovered that Uranus' magnetic field doesnot follow the usual north-south axis found on the otherplanets. Instead, the field is tilted 60 degrees and offsetfrom the planet's center, a phenomenon that on Earth wouldbe like having one magnetic pole in New York City and theother in the city of Djakarta, on the island of Java in Indone-

sia.Uranus' atmosphere consists mainly of hydrogen, with

some 12 percent helium and small amounts of ammonia,methane and water vapor. The planet's blue color occursbecause methane in its atmosphere absorbs all othercolors. Wind speeds range up to 580 kilometers (360 miles)per hour, and temperatures near the cloud tops average-221 degrees Celsius (-366 degrees Fahrenheit).

Uranus' sunlit south pole is shrouded in a kind of photo-chemical "smog" believed to be a combination of acetylene,ethane and other sunlight-generated chemicals. Surround-ing the planet's atmosphere and extending thousands ofkilometers into space is a mysterious ultraviolet sheenknown as "electroglow."

Uranus' clouds am vktuafiy featureless unlike those of ffte othergiant, gaseous planets in the outer solar system. Both picturesare from Voyager 2 Me left image is unenhanced; the rightenhances contrasts to highlight zonal bands In the atmosphere.The occasional, smell doughnut shapes are out-of-focus dustparffcies on the camera tens.

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Miranda Is characterized by variations in tensin. The icecliffs (upper right) are 20 kilometers (12 miles) high.

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Uranus' rings, which appear dm* from Earth, we brightened bysunlight passing through them In this photograph taken byVoyager 2. Parts of the rings are smeared and stars register asstreaks in this long exposure.

Approximately 8,000 kilometers (5,000 miles) belowUranus' cloud tops, there is thought to be a scalding oceanof water and dissolved ammonia some 10,000 kilometers(6,200 miles) deep. Beneath this ocean is an Earth-sizedcore of heavier materials.

Voyager 2 discovered 10 new moons, 16-169 kilometers(10-105 miles) in diameter, orbiting Uranus. The fivepreviously known Miranda, Ariel, Umbriel, Titania andOberon range in size from 520 to 1,610 kilometers(323 to 1,000 miles) across. Representing a geologicalshowcase, these five moons are half-ice, half-rock spheresthat are cold and dark and show evidence of past activity,including faulting and ice flows.

The most remarkable of Uranus' moons is Miranda. Itssurface features high cliffs as well as canyons, crater-pocked plains and winding valleys. The sharp vadations interrain suggest that, after the moon formed, it was smashedapart by a collision with another body an event notunusual in our solar system, which contains many objectsthat have impact craters or are fragments from largeimpacts. What is extraordinary is that Miranda apparentlyreformed with some of the material that had been in itsinterior exposed on its surface.

Uranus was thought to have nine dark rings; Voyager 2imaged 11. In contrast to Saturn's rings, which are com-posed of bright particles, Uranus' rings are primarily madeup of dark, boulder-sized chunks,

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Neptune

Voyager 2 completed its 12-year tour of the solar systemwith an investigation of Neptune and the planet's moons.On " ugust 25, 1989, the spacecraft swept to within4,850 kilometers (3,010 miles) of Neptune and then flewon to the moon Triton. During the Neptune encounter, it

became clear that the planet's atmosphere was moreactive than Uranus',

Voyager 2 observed the Great Dark Spot, a circularstorm the size of Earth, in Neptune's atmosphere. Resem-bling Jupiter's Great Red Spot, the storm spins counter-clockwise and moves westward at almost 1,200 kilometers(745 miles) per hour. Voyager 2 also noted a smaller darkspot and a fast-moving cloud dubbed the "Scooter," aswell as high-altitude clouds over the main hydrogen andhelium cloud deck. The highest wind speeds of any planetwere observed, up to 2,400 kilometers (1,500 miles) perhour.

Like the other giant planets, Neptune has a gaseoushydrogen and helium upper layer over a liquid interior.The planet's core contains a higher percentage of rockand metal than those of the other gas giants. Neptune'sdistinctive blue appearance, like Uranus' blue color, isdue to atmospheric methane.

Neptune's magnetic field is tilted relative to the planet'sspin axis and is not centered at the core. This phenome-non is similar to Uranus' magnetic field and suggests thatthe fields of the two giants are being generated in an areaabove the cores, where the pressure is so great that liquidhydrogen assumes the electrical properties of a metal.Earth's magnetic field, on the other hand, is produced byits spinning metallic core and is only slightly tilted andoffset relative to its center.

Voyager 2 also shed light on the mystery of Neptune'srings. Observations from Earth indicated that there werearcs of material in orbit around the giant planet. It was notclear how Neptune could have arcs and how these couldbe kept from spreading out into even, unclumped rings.Voyager 2 detected these arcs, but they were, in fact, partof thin, complete rings. A number of small moons couldexplain the arcs, but such bodies were not spotted.

Astronomers had identified the Neptunian moons Tritonin 1846 and Nereid in 1949. Voyager 2 found six more.One of the new moons Proteus is actually larger thanNereid, but since Proteus orbits close to Neptune, it waslost in the planet's glare for observers on Earth.

Triton circles Neptune in a retrograde orbit in under sixdays. Tidal forces on Triton are causing it to spiral slowlytowards the planet. In 10 to 100 million years (a short timein astronomical terms), the moon will be so close thatNeptunian gravity will tear it apart, forming a spectacularring tc, lccompany the planet's modest current rings.

Triton's landscape is as strange and unexpected asthose of lo and Miranda The moon has more rock than its

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counterparts at Saturn and Uranus. Triton's mantle isprobably composed of water-ice, but its crust is a thinveneer of nitrogen and methane. The moon shows twodramatically different types of terrain: the so-called "canta-loupe" terrain and a receding ice cap.

Dark streaks appear on the ice cap. These streaks arethe fallout from geyser-like volcanic vents that shoot nitro-gen gas and dark, fine-grained particles to heights of2-8 kilometers (1-5 miles). Triton's thin atmosphere, only1/70.000th as thick as Earth's, has winds that carry thedark particles and deposit them as streaks on the icecap the coldest surface yet discovered in the solarsystem (-235 degrees Celsius, -391 degrees Fahrenheit).Triton might be more like Pluto than any other objectspacecraft have so far visited.

The Great Dar* Spot (top), the dotal named "Scooter" (middle)and Dark Spot 2 (bottom) with its bright core am the mostprominent features In Neptune's atmosphere.

This photograph overexposfd Neptune (appearing as a crescent)to highlight the thme dumps In the planets outer ring.

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Pluto

A possible future mission to Pluto (right) and its moon Charon isshown in this artisrs rendering.

Pluto is the most distant of the planets, yet the eccentricityof its orbit periodically carries it inside Neptune's orbit,where it has been since 1979 and where it will remain untilMarch 1999. Pluto's orbit is also highly !!)clined tilted17 degrees to the orbital plane of the other planets.

Discovered in 1930, Pluto appears to be little more thana celestial snowball. The planet's diameter is calculated tobe approximately 2,300 kilometers (1,430 miles), only two-thirds the size of our Moon. Ground-based observationsindicate that Pluto's surface is covered with methane iceand that there is a thin atmosphere that may freeze and tallto the surface as the planet moves away from the Sun.

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Observations also show that Pluto's spin axis is tipped by122 degrees.

The planet has one known satellite, Charon, discoveredin 1978. Charon's surface composition is different fromPluto's: the moon appears to be covered with water-icerather than methane ice. Its orbit is gr avitationally lockedwith Pluto, so both bodies always keep the same hemi-sphere facing each other. Pluto's and Charon's rotationalperiod and Charon's period of revolution are all 6.4 Earthdays.

Although no spacecraft have ever visited Pluto, NASA iscurrently exploring the possibility of such a mission.

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Com ets

The outermost members of the solar system occasionallypay a visit to the inner planets. As asteroids are the rockyand metallic remnants of the formation of the solar system,comets are the icy debris from that dim beginning and cansurvive only far from the Sun. Most comet nuclei reside inthe Oort Cloud, a loose swarm of objects in a halo beyondthe planets and reaching perhaps halfway to the neareststar.

Comet nuclei orbit in this frozen abyss until they aregravitationally perturbed into new orbits that carry themCi0V3 to the Sun. As a nucleus falls inside the orbits of theouter planets, the volatile elements of which it is madegradually warm; by the time the nucleus enters the regionof the inner planets, these volatile elements are boiling. Thenucleus itself is irregular and only a few miles across, andis made principally of water-ice with methane and ammo-nia materials very similar to those composing the moonsof the giant planets.

As these materials boil off of the nucleus, they form acoma or cloud-like "head" that can measure tens ofthousands of kilometers across. The coma grows as thecomet gets closer to the Sun. The stream of chargedparticles corning from the Sun pushes on this cloud,blowing it back like a flag in the wind and giving rise to thecomet's "tails." Gases and ions are blown directly backfrom the nucleus, but dust particles are pushed moreslowly. As the nucleus continues in its orbit, the dustparticles are left behind in a curved arc.

Both the gas and dust tails point away from the Sun; ineffect, the comet chases its tails as it recedes from theSun. The tails can reach 150 million kilometers (93 millionmiles) in length, but the total amount of material containedin this dramatic display would fit in an ordinary suitcase.Comets from the Latin cometa, meaning "Iong-haired"

are essentially dramatic light shows.Some comets pass through the solar system only once,

but others have their orbits gravitationally modified by aclose encounter with one of the giant outer planets. Theselatter visitors can enter closed elliptical orbits and repeat-edly return to the inner solar system,

30

Halley's Comet is the most famous example of a rela-tively short period comet, returning on an average of onceevery 76 years and orbiting from beyond Neptune to withinVenus' orbit. Confirmed sightings of the comet go back to240 B.C. This regular visitor to our solar system is namedfor Sir Edmund Halley, because he plotted the comet'sorbit and predicted its return, based on earlier sightingsand Newtonian laws of motion. His name became part ofastronomical lore when, in 1759, the comet returned onschedule. Unfortunately, Sir Edmund did not live to see it.

A cornet can be very prominent in the sky if it passescomparatively close to Earth. Unfortunately, on its mostrecent appearance. Halley's Comet passed no closer than62.4 millioq kilometers (38.8 million miles) from our world.The comet was visible to the naked eye, especially forviewers in the southern hemisphere, but it was not spec-tacular. Comets have been so bright, on rare occasions,that they were visible during daytime. Historically, cometsightings have been interpreted as bad omens and havebeen artistically rendered as daggers in the sky.

The Comet Rendezvous/Asteroid Flyby (CRAF) space-craft will become the first traveler to fly close to a cometnucleus and remain in proximity to it as they both approachthe Sun. CRAF will observe the nucleus as it becomesactive in the growing sunlight and begins to have its lighterelements boil off and form a coma and tails. Severalspacecraft have flown by comets at high speed; the firstwas NASA's International Cometary Explorer in 1985. Anarmada of five spacecraft (two Japanese, two Soviet andthe Giotto spacecraft from the European Space Agency)flew by Halley's Comet in 1986.

3 .)

Halley's Comet, the most famous of all owlets, lest visited theinner solar system in 1988. It will return again in MI.

3 4

BEST COPY AVAI

31

The Planets at a Glance

k40 AO' e

4,. 410 4111

0,413

on.

Mercury Venus Earth Mars . Jupiter Saturn Uranus Neptune Pluto

Mean DistanceFrom Sun

Millions of 57 9 108 2 149 6 227.9 778 3 1.429Kilometers

Millions of 36 67 2 93 141.6 483.6 888 2Miles

Period of Rotation* 58 65days

Inclination of Orbit 7.0to Ecliptic (Degrees)

243 01days,retrograde

4le! ,

d 4

3 39

23 hours. 24 hours,56 min 37 mm.

9 hours,56 min.

2 8 /5 4,504 5.900

1.786 2,799

10 hours. 17 hours,40 min. 14 mm.

16 hours, 6.39 days,6 min. relrograde

0.0 1.85

' r:'a *.' ',1',..7, i bp)

-di

1.31 2.49 0.77 1.77 17.15

$-17477"--.777777-7, :TT?

t'Or ts!

fC'

.

Equatorial Diameter

Kilometers

Miles

t yU3,

ikt.!41:

-4 V,

Satellites

4.879 12,104 12,755 6,790 142.796 120,660 51,118 49,528 2.300 (Appx )

3,031 7,521 7,926 4.219 98.729 74,975 31,763 30,775 1.429 (Appx.)

None None

'Periods are given in Earth lime.

32

157177:

ktiaataaAt;>. "

2 16 18 15 8

espite their efforts to peer across the vast

distances of space through an obscuring

atmosphere, scientists of the past had only

one body they could study closely Earth,

But since 1959, spaceflight through the solar system has

lifted the veil on our neighbors in space.

We have learned more about our solar system and its

members than anyone had in the previous thousands of

years. Our automated spacecraft have traveled to the

Moon and to all ffie planets beyond our world except Pluto;

they have observed moons as large as small planets, flown

by comets and sampled the solar environment. Astronomy

books now include detailed pictures of bodies that were

on6, smudges in the largest telescopes for generations.

We are lucky to be alive to see these strange and beautiful

places and objects.

The knowledge gained from our journeys through the

solar system has redefined traditional Earth sciences like

geology and meteorology and spawned an entirely new

discipline called comparative planetology. By studying the

geology of planets, moons, asteroids and comets, and

comparing differences and similarities, we are learning

more about the origin and history of these bodies and the

solar system as a whole.

We are also gaining insight into Earth's complex weather

systems. By seeing how weather is shaped on other worlds

and by investigating the SUr7's activity and its influence

throughout the solar system, we can better understand

climatic conditions and processes on Earth

We will continue to learn and benefit as our automated

spacecraft explore our neighborhood in space. One

current mission is mapping Venus; others are flying

between worlds and will reach the Sun and Jupiter after

complex trajectory ac#ustments. Future missions are

planned for Mars, Saturn, a cornet and the asteroid belt.

We can also look forward to the time when humans vvill

once again set foot on an alien world. Although astronauts

have not been back to the Moon since December 1972,

plans are being formulated for our return to the lunar

landscape and for the human exploration of Mars and even

the establishment of martian outposts. One day, taking a

holiday may mean spending a week at a lunar base or a

martian colony!

National Aeronautics andSpace Administranon

Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena. Ca!ifornia

JP1. 410-34-1 6091

3 7